The present invention relates to a battery pack. More specifically, the present invention relates to a battery pack in which the volume of an exterior portion thereof has been decreased to a smallest possible value.
Recently, a demand for information equipment, such as a notebook personal computer, mobile communication equipment, such as a portable telephone set, or a portable electronic equipment, such as a video camera, is increasing rapidly. As the power supply for these electronic equipment, hermetically sealed small-sized secondary batteries, such as a nickel-cadmium battery, a nickel-hydrogen battery or a lithium ion battery, are predominantly used. Of these, the lithium ion battery is used in a large variety of applications, by exploiting its characteristics, including high voltage, high energy density and light weight.
In particular, as measures for combating the problem of liquid leakage, presented in case of employing a liquid type electrolyte, a so-called polymer lithium ion secondary battery, employing a high molecular gel film, obtained on impregnating a polymeric material with a non-aqueous electrolytic solution, or employing an all-solid electrolyte, as an electrolyte, has been proposed.
In a battery of this type, a unit cell 101, comprised of a battery device of a laminated structure, is housed within upper and lower halves 103, 104 of a plastics casing, along with a connection substrate, including a protective circuit or terminals, to constitute a battery pack 100, as shown for example in FIG. 18 (Japanese Laying-Open Patent Publication 2002-8606).
With such structure, a thickness of 0.3 to 0.4 mm is needed for the plastics casing. If a double-face tape for mounting and/or the tolerance is taken into account, an increase in thickness on the order of 0.8 to 1 mm needs to be considered for the cell thickness. As regards the outer rim, a shape allowance to permit ultrasonic welding of upper and lower casing halves is needed, and hence a thickness on the order of 0.7 mm is required. The result is that the volume of battery pack is necessarily increased by a factor of 1.3 to 1.4.